I have a 2004 Roadster which I purchased new in 2006, and a few months ago I had a rattle appear, took it to the dealer, and they found a Cat Converter had failed, and was rattling inside. They apparently had to replace 3 of the 4, something to do with the fact they were laser welded. (I didn't really care so long as it was fixed under warranty).

I was led to believe by the service manager, that it was a quality issue with the converter itself. My car had done about 15,000 kms when the converter went. The service manager told me that if I had to pay for the job, it would be around AUD $2,500, which converts to 1,050 pounds, or US $2,134., not cheap in anyone's money!!